The Bible says that when your brother dies, you take care of his wife and family. It never says polygamy is fine in the eyes of God.
It does however say you can see your daughter into sexual slavery.
Levitical Law should really just be taken out of the Bible. If you read the first five books of the Bible, you see a story of God talking directly to God, and man distancing himself by asking for a King, wanting Priests, laws, formalized religion, etc.
There is no real indication that Levitical Law is the Law of God, but more so really the Law of Ancient Judaism and somewhat the Law of Modern Judaism.